The field of the invention is magnetic resonance imaging (MRI) and systems. More particularly, the invention relates to a device and method for imaging multiple regions of interest (ROIs) using a radiofrequency (RF) coil.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MRI-based techniques are increasingly preferred over other imaging modalities in clinical medicine, for example, due to growing healthcare concerns over cumulative exposure to ionizing radiation, as is used in x-ray and computed tomography (CT) imaging. Even in dentistry applications, where x-ray has been a mainstay of clinical practice, MRI is growing in popularity.
Beyond the avoidance of ionizing radiation, MR techniques for diagnostic imaging in dentistry have become of greater interest due to new advances that have enabled direct imaging of densely calcified tissues of the human body, such as dentin and enamel. These tissues have low water content and, thus, a low fraction of protons to obtain signal from for MRI imaging. Also, these tissues have a quickly decaying signal and, thus, very short transverse relaxation times, T2. In other words, the signal from mineralized dental tissue decays before MRI signal digitization occurs, resulting in MRI images with little or no image intensity. However, currently, there are at least four different and clinically viable MRI methods for obtaining images of densely calcified dental tissues, these include: i) Ultrashort TE (UTE), ii) Sweep Imaging with Fourier Transformation (SWIFT), iii) FID-projection imaging also called BLAST, RUFIS, WASPI, or zero TE (ZTE), and iv) combined PETRA techniques. Thus, dental MRI is a feasible technique for diagnostic imaging.
Dental MRI can be more informative than x-ray imaging techniques by visualizing, noninvasively and simultaneously, both hard and soft tissues in three dimensions. However, clinical MRI has yet to attain the resolution of CT imaging and, in particular cone beam CT (CBCT) imaging from 0.1-0.3 mm for all the regions of interest, such as for all teeth required for dental applications. The SNR and resolution for dental MRI is highly dependent on the configuration (i.e., filling factor) and performance of the radio frequency (RF) coil and field-of-view (FOV). For example, to image the mandible, extra-orally positioned surface coils, or head coils, are required, which have limited resolution and sensitivity and are expensive. As such, traditionally, MRI imaging has been restricted to imaging of the mandibular neurovascular bundle, vitality of the pulp structure, visualization of the anatomy and pathology of the dento-alveolar region, detection of osteomyelitis in the mandible, and the indirect imaging of highly mineralized tissue through contrast produced by an MRI-visible medium.
Also, in order to reach the necessary resolution, artifacts related to patient movement need to be minimized (including avoiding swallowing). Therefore the dental coil needs to be rigidly fixated in relation to the imaging system yet comfortable to prevent patient fatigue that results in fidgeting. The specifics of short T2 imaging do not allow for the use of slice or slab selections, both to preserve signal from hard tissues having ultra-short T2 and due to the three-dimensional radial free-induction decay acquisition strategy. Thus the acquired FOV must include the entire sensitive volume of the RF coil to avoid signal folding onto areas of interest in the image. The spatial resolution, which is the linear size of the image voxel, depends on the FOV and the reconstructed matrix size. To reach the necessary resolution, for example, 0.3 mm, the FOV should not exceed about 80 mm to 120 mm with 2563 to 3843 matrix sizes, respectively. However, increasing the matrix size is not practical because it is restricted by the clinical scan time, typically around 2-3 minutes for conventional 3D scanning. Thus, in dental applications using traditional head or neck coils, the necessary resolution will not be achieved. Therefore, a dedicated, localized surface coil design is needed.
The logical approach to imaging teeth would be to adopt existing surface coil designs with extra-oral placement adjacent to the area of interest. The diameter of such a receive coil should not exceed about 120 mm because it is limited by the size of the optimal FOV. The depth of the sensitive region in the axial direction, which is perpendicular to the plane of the surface coil, is limited to about the radius of the coil. To obtain an image of a right molar tooth, for example, such an extraoral coil could be positioned over the right cheek. For an average-sized patient, the distance between the coil and the molar teeth is between 30 and 50 mm, and as a result, sensitivity is significantly diminished. In addition, with the coil in this configuration, the cheek and buccal fat produce intense signals. Therefore, the resulting images contain more signals from less important structures of the mandible and maxilla and vice versa.
The resolution and signal-to-noise ratio (SNR) could be increased by using a loop coil positioned intra-orally, in the buccal vestibule that is between the teeth and adjacent cheek. By sacrificing some comfort, as well as some SNR, the intense signal from the cheek can be shielded out. However, due to space restriction and the need for comfortable positioning of the coil, the root tips of the teeth are unavoidably outside the coil sensitive volume and not well visualized. Further, normal intraoral anatomy makes it difficult to position the coil posterior enough to obtain images of the most distal teeth in the mouth, and common variations of intraoral anatomy, such as the presence of buccal tori and frena, pose additional difficulties in positioning the coil. This suggests that using the buccal vestibule approach for RF coil placement is problematic for patient comfort and limits visualization of oral structures. Accommodating all patient sizes and anatomical variations would likely require multiple types and sizes of coils as well as repeated scans in order to obtain needed images
Dental MRI continues to develop as an important imaging technique, however, it would be desirable to have a system and method for placement of a dental coil such that the sensitive volume of the coil covers the most important dental structures and excludes the less informative tissues (i.e., the cheeks, lips and tongue), as well as reduce patient discomfort and total time to image the areas of interest.